Table of Contents

Brønsted and Lowry Definitions

An acid is a proton donor and a base is a proton acceptor. For example a hypothetical acid, HA dissociates into H+ and A-

HA ↔ H+ + A-

An acid, a proton donor, donates protons in water forming hydronium ions (protonated water, H3O+). A base, proton acceptor, removes protons from water forming hydroxide ions (deprotonated water, OH-) Using curved arrows to demonstrate the mechanism by which a proton is transferred from hydrochloric acid to the base water. The arrows demonstrate the movement of electrons but the key feature of the Bronsted Lowry reaction is the transfer of protons.

A lone pair from the base creates a new bond with an acidic proton and the electron pair originally linking the proton to the remainder of the acid shifts to becomes a lone pair on the departing conjugate base.

Estimating Relative Acid Base Strengths

Relative strength of an acid (HA) and weakness of conjugate base can be estimated using three structural properties:

Electronegativity, and thus acidity, increases as we go left to right across the periodic table. Great electronegativity results in a more acidic proton, and the more polar bonds.

Molecules get larger as we go down a column in the periodic table. Dissociation of a large A is favored because the 1s hydrogen and larger outer-shell overlap is poor, creating a weak H-A bond. This can be further explained through electron electron repulsion, which is reduced when outer shell orbitals allow electrons to occupy a greater volume of space.

Delocalization of charge onto several atoms is accomplished via resonance of A-. The presence of additional electronegative atoms in A- increases the effect.

Summary: Basicity of A- decreases to the right and down the periodic table, acidity of HA increases to the right and down the periodic table.

Several molecules have the ability to act as acids or bases under differing conditions, thus they are Amphoteric.

Lewis acids and bases interact by sharing an electron pair

Lewis base share its lone pair electrons with a lewis acid to form a new covalent bond, thus can be expressed by an arrow moving in the direction of electron movement (base to acid)

Electrophiles and Nucleophiles interact through movement of an electron pair

Processes that exhibit very similar characteristics as acid-base reactions and are described using the same electron pushing arrows.

Electrophile "electron loving": An electron deficient atom, ion or molecule that has an affinity for an electron pair and will bond to a base or nucleophile. (all lewis acids are electrophiles)

Nucleophile "nucleus loving": An atom ion or molecule that has an electron pair that may be donated in bonding to an electrophile or lewis acid. (all nucleophiles are lewis bases)

The diagram below demonstrates the flow of electrons using electron pushing arrows:

Haloalkanes (compounds with carbon-halogen bonds) are general nucleophilic substitution reactions. Despite differing halogens and arrangement of substituents , all arrangements/combinations behave similarly allowing us to conclude that it is the actual presence of the carbon-halogen bond that controls the behavior of the haloalkane. The C-X bond is the functional group/controlling factor of reactivity.

Problems

Which is a stronger acid H2O or H2S? Which is a stronger base, HO- or HS-?

Rank the following in order of increasing acidity. CH4, HF, H2O, NH3. Provide each acids conjugate base?